Process for the generation and utilization of furnace atmospheres for the heat treatment of metals, especially of steel



NOV. 26, 1968 w, GOEHRING 3,413,161

PROCESS FOR THE GENERATION AND UTILIZATION OF FURNACE ATMOSPHERES FORTHE HEAT TREATMENT OF METALS, ESPECIALLY OF STEEL Filed Sept. 19, 1966INVENTOR WERNER GOEHRING Attorney United States Patent 6 Claims. (Cl:148-16.5)

ABSTRACT OF THE DISCLOSURE Hydrocarbon fuel is mixed with a quantity ofair less than that required for complete combustion. This mixture isburned, while steps are taken to prevent the escape of generated heatfrom the burning mixture. This generated heat is accumulated untilchemical equilibrium of the products of combustion is reached at atemperature above the processing temperature of the steel to be treated.The steel is then exposed to the products of combustion for earburizing,bright annealing and oxidation-free cooling.

Cross-reference to related application This is a continuation-in-partapplication to copending parent application Ser. No. 396,980 whichmatured into U.S. Patent No. 3,290,030, on Dec. 6, 1966.

Field of the invention The present invention relates to a process forgenerating and using furnace atmospheres for the heat treatment ofsteel.

Summary of the invention The objects of the invention are to provide aprocess:

For producing high-quality furnace atmospheres from fuels consistingessentially of hydrocarbons;

For producing atmospheres that affect the carbon content of the steelcompletely according to the formation and decomposition of CO on themetal surface;

For producing atmospheres that are in equilibrium and which cantherefore be judged for their carburizing effect by the measurement ofonly one gas component;

For producing a furnace atmosphere without the necessity of removing COand H 0, said atmosphere therefore being available immediately aftercombustion for contact with the metal to be treated and therefore alsobeing usable for regeneration heat transfer.

For producing atmospheres that can be used even at low heat treatingtemperatures and that prevent oxidation, even during cooling of workpieces;

For producing furnace atmospheres without the necessity of speciallyheating the combustion chamber for the fuel-air-mixture;

For producing high-quality furnace atmospheres from combustion gases ofhydrocarbon fuels without any preliminary treatment, which atmospherescan be used for bright annealing of steel and subsequent cooling Withoutany oxidation of the work pieces.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description.

The only figure of the drawing is an equilibrium chart.

In outline, the steps of the process of the invention comprise mixingwith a hydrocarbon fuel a quantity of air less than that needed for itscomplete combustion, burning the air-fuel mixture, preventing the escapeof 3,413,16 1 Patented Nov. 26, 1968 heat from said burning mixture andfrom the resulting gaseous products of combustion until said productsreach chemical equilibrium at a temperature above the processingtemperature of the metal to be treated, collecting said products andexposing said metal to said products. As will be seen in the followingparagraphs, other steps are added to these, depending on the particularadvantages desired.

Description 0 the preferred embodiments The various types of furnacesconventional in the are suitable for the performance of the process ofthis invention. The drawings of the furnaces of the copending parentcase and their descriptions are hereby incorporated by reference onlyfor the purpose of showing two furnaces in which the process can beperformed. The process is, of course, not limited to its performance inthese two furnaces.

Hydrocarbons, the molecules of which consist only of carbon andhydrogen, or fuels essentially containing these hydrocarbons are burnedin a combustion chamber which is connected with the processing chamberof the metal work pieces. The combustion chamber is protected againstheat losses and is of such small volume that the gaseous product ofcombustion, hereinafter referred to as combustion gases, reach a highcombustion temperature and assume a state of chemical equilibrium abovethe temperature of the material to be treated. Thus, these gasesinfluence the carbon content of the steel exclusively by the combustiongas component CO and/ or prevent any oxidation during annealing andsubsequent cooling of the steel. Chemical equilibrium means that therelative amounts of the various products of combustion at a giventemperature are neither increasing nor decreasing.

As a rule, in comparison to other flowable tools as for instance cokeoven gas, the hydrocarbons contain little hydrogen in proportion tocarbon. They therefore bring the advantage that during the combustion,combustion gases low in Water vapor are generated. Although thecombustion may take place under conditions of insufficient air forgeneration of the required content of combustion-gas components whichhave a reducing effect, namely H and CO, hydrocarbons still permit arelatively high feeding of air, since they have no combined oxygen. Thismakes the theoretical combustion temperature in many cases higher thanthe conventional processing temperatures of the work pieces. In the caseof combustion of the hydrocarbon-air mixture in a small combustionchamber, the volume of which is designed no larger than required for themovement of the combustion reaction to chemical equilibriurru a highcombustion temperature occurs. Combustion is effected preferably in thepresence of reaction accelerating, catalytic filling materials, such asnickel oxide or platinum black. The high temperature attained is due toheat reflection from the adjacent cham ber surfaces onto the burninggases. Due to the small surface and the thermal protection of thiscombustion chamber as well as possibly through the structuralunification with the chamber of the work pieces, the heat losses can bekept so low that the combustion gases almost reach the theoreticalcombustion temperature. Then, due to the high reaction velocity at ahigh combustion temperature, they assume the state of equilibrium.

In the case of employing these combustion gases for carburizing workpieces, in accordance with Boudouards reaction 2CO CO +C, the transferof carbon takes place exclusively through the dissociation of the carbonmonoxide, more stable at higher temperatures, at the cooler surface ofthe work pieces. Because of the attained state of equilibrium thecomposition of the combustion gases permits an unambiguous conclusion tobe made concerning the carburizing effect. This conclusion can be madeby the measurement of one single combustion-gas component. This isusually the water vapor and is determined by means of measurement of thedew point.

In the case of the bright annealing of steel, cooling work pieces arenot oxidized by combustion gases that are generated in the manner of theprocess, even though they have been generated with a relatively highfeeding of air. The gases contain more Water vapor in proportion tohydrogen than is permissible for oxidation-free cooling according toteachings of the prior art. Obviously, this oxidation is avoided throughthe high content of carbon monoxide that is present due to thecomposition of the fuel. Here, it is likely that water vapor reactslocally with carbon monoxide under the catalytic influence of the Workpiece surfaces to reach the state of equilibrium that corresponds to themomentary temperature of the metal material; at this state ofequilibrium the gases then have a reducing effect in accordance with theknown values.

In a further development of the process according to the invention, airor a hydrocarbon-air mixture is introduced into the combustion chambervia a preheater. This supplies the combustion components with an amountof heat which, together with the amount of heat released duringcombustion, provides for the required high reaction temperature. Forthis purpose the heating installation of the processing chamber as wellas the heat of the cooling work pieces of continuously fed furnaces canbe used. Regeneration heat transfer using previously charged gases ofthe atmosphere is also applicable.

It has been found that, in the case of a hydrocarbon-air mixture whichflows faster than its combustion velocity, no precornbustion and, aboveall, no cracking connected with undesirable separation of carbon occurswithin the preheater. This fact is based upon the circumstance that thecarbon of the hydrocarbon, which disassociates upon strong heating, addsin statu nascendi to the oxygen present in the mixture. Thus, onlygaseous products of cracking are brought about; these burn free of sootin the combustion chamber.

The purpose of the preheating of the combustion components is, on theone hand, to reach, in the case of some hydrocarbons, the required hightheoretical combustion temperature, especially in the case of the small4 on the H/C ratio of the fuel. The figure shows the fa miliar iron/ironoxide equilibrium curve in a mixture of CO CO, H 0, and H at differenttemperatures. The combustion gases from fuels, as entered on the chart,form curves because their composition changes as a function oftemperature according to the reaction where k the lack of air (ratio ofthe air fed to the air needed for complete combustion);

a=the desired CO /CO ratio;

b=the desired H O/H ratio;

x=the H/ C ratio of the fuel.

If bright annealing and oxidation-free cooling of steel is desired, thecombustion gas curve must be located some certain distance from theiron/iron oxide equilibrium curve in the reducing region of the chart.For instance, if the choices a=1.1 and b=0.025 are made for the lowtemperature region, the required amount of air follows the equation:

For the choices a=0.25 and b:0.55 made for the high temperature region,the required amount of air is:

In the case of fuels having an H/ C ratio greater than 1.8, thecomposition curve of the combustion gas comes closest to the iron/ ironoxide curve in the region of lower temperatures. Therefore, here theEquation 1 must be used.

In the case of fuels with H/C smaller than 1.8 (for example, benzene),the closest approach is in the region of higher temperatures. Thisdictates use of Equation 2.

TABLE I.TABLE OF EXAMPLES Lack of air Required preheating in (air fed inComposition of the combustion gases order to reach the proportionTheoretical (at 1,l00 C. in a state of equilibrium), percent theoreticalcombustion Type of heat Fuel to amount combustion temperature of 1,1000.

treatment (hydrocarbons) of air in the temperature,

case of 0. Air Mixture complete CO2 00 E20 Hz N2 preheating, preheating,combustion) 0. 0.

Bright annealing and Benzene vapor 0.55 1, 620 5. 0 19. 5 4.15 8.1 63.25 None None oxidation-free a s. cooling of steel. Light fuel oil 0. 521, 300 4. 3 16. 2 6. 9 13.0 59. 6 None None Propane C3H5 0. 46 1,230 3.016. 0 7.0 18. 3 55. 7 None None Methane CH4 0.4 975 2. 1 14.5 7. 7 25. 550. 2 220 160 Cementatiou or de Benzene vapor O. 4 1, 180 29. 5 Trace14. 75 55. 75 None None carburization-free CGHG. Trace annealing ofsteel. Propane O Hg 0.3 600 Trace 23. 7 Trace 31.6 44. 7 890 590permissible feeding of air for the production of combus- 0 Table I glvesexamples of fuels for use in the process tion gases with a carburizingeffect. For the production of combustion gases that only preventoxidation, in the case of which a larger feeding of air is permissible,the processing chamber can be directly heated by means of the hotcombustion gases. In the case of the employment of a heatinginstallation, it is expedient to use it only for the preheating of theprocessed material, since then it is subject to a smaller thermalstress.

By means of the process according to the invention it is thus feasibleto generate furnace atmospheres for bright-annealing and bright-coolingpurposes for as well as for cementation-free annealing, and also forcarburization of steel in a simple furnace installation.

The amount of air to be used during combustion depends on the desired CO/CO and HgO/H ratios and of the present invention. Other fuels can beanalyzed for application using the above equations.

The process according to the invention can be employed at all furnaceinstallations suitable for protective-gas operation with burned gases.

It should be understood, of course, that the foregoing disclosurerelates to only preferred embodiments of the invention and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

The various types of furnaces conventional in the art can be used forthe performance of the process of this invention. The drawings of thefurnaces of the copending parent case and their descriptions have beenincorporated herein only for the purpose of illustrating two furnaces inwhich the process of the invention can be performed. The invention ishowever not limited to performance in these furnaces.

I claim:

1. A process for the production and utilization of furnace atmospheresfor carburizing, bright annealing and oxidation-free cooling of steel,comprising the steps of: mixing with a hydrocarbon fuel a quantity ofair less than that required for complete combustion, burning the airfuelmixture, preventing the escape of heat from said burning mixture andfrom the resulting gaseous products of combustion, raising thetemperature of said gaseous products of combustion to a temperatureabove the processing temperature of the steel to be treated, bringingsaid gaseous products to chemical equilibrium at said temperature abovethe processing temperature, both the steps of raising and bringing beingaccomplished only by burning said air-fuel mixture, without supplementalexternal heating of said gaseous products, collecting said products andexposing the steel to said products.

2. A process as in claim 1 including the step of preheating said airprior to said step of burning.

3. A process as in claim 1 including the step of pre heating said airand a part of said mixture prior to said step of burning.

4. A process as claimed in claim 2, said step of preheating includin thestep of transferring heat from said products.

5. A process as claimed in claim 3, said step of preheating includingthe step of transferring heat from said products.

6. A process as claimed in claim 1, said step of burning including thestep of contacting said air-fuel mixture with a reaction acceleratingmaterial.

References Cited UNITED STATES PATENTS 2,763,476 9/1956 Ness et a1.148-465 X 2,763,582 9/1956 Rusciano 14816.5

2,799,490 7/1957 Rusciano 14816.5 X

2,886,303 5/1959 Rusciano 14816.5 X

CHARLES N. LOVELL, Primary Examiner.

